1. Field of the Invention
The present invention relates to a prepolymerization process for producing a conjugated diene compound prepolymer solution. More particularly, the present invention relates to a prepolymerization process for producing a conjugated diene compound prepolymer solution and a process for producing a vinyl aromatic compound high impact polymer material from the above-mentioned prepolymer solution.
2. Description of the Related Art
It is known that the impact resistance of vinyl aromatic hydrocarbon polymers, for example, styrene polymers, which are hard and fragile, can be improved by adding various types of toughening agents consisting of non-vulcanized rubber materials to the polymers. Particularly, high impact styrene polymers which have been produced by a bulk polymerization or bulk-suspension polymerization of a styrene compound monomer in the presence of a non-vulcanized rubber material are cheap have a high processability and various excellent physical properties, and thus are useful for various industries. The non-vulcanized rubber materials usable for the above-mentioned purpose are usually selected from butadiene polymers and styrene-butadiene copolymers. Especially, when a high impact property at a low temperature is required, various types of butadiene polymers, for example, low cis-polybutadiene rubbers produced by an anionic polymerization method in the presence of a catalyst consisting of an organic lithium compound either alone or comprising, the organic lithium compound as the major component, or high cis-polybutadiene rubbers produced by a polymerization method using a coordination catalyst comprising, as a principal component, a transition metal, for example, cobalt, nickel or titanium, are used as a toughening agent for various purposes.
The above-mentioned butadiene polymers and styrene-butadiene copolymers are produced by a polymerization process using an inert solvent, and are used as a toughening agent after the inert solvent is removed from the polymers or copolymers.
Also, in the production of a high impact vinyl aromatic hydrocarbon polymer, a toughening agent consisting of a rubber material is dissolved in a corresponding vinyl aromatic hydrocarbon monomer, for example, styrene monomer, and the resulting solution is subjected to a bulk or bulk-suspension polymerization process. The above-mentioned dissolving procedure requires much labor and an effective apparatus for evenly dissolving the rubber material in the monomer.
To eliminate the above-mentioned disadvantages, Japanese Unexamined Patent Publication (Kokai) No. 57-149316 and U.S. Pat. No. 4,311,819 disclose a process for producing a solution of a rubber-like butadiene prepolymer in a medium consisting of an vinyl aromatic hydrocarbon compound and then for producing a high impact butadiene-vinyl aromatic hydrocarbon copolymer from the prepolymer solution. This process is however, disadvantageous in that, since the butadiene is prepolymerized in the presence of a lithium-based catalyst in the vinyl aromatic hydrocarbon compound medium, the resultant butadiene prepolymer contains, in the prepolymer chains thereof, about 10% by weight or more of the vinyl aromatic hydrocarbon groups copolymerized with butadiene, and thus exhibit an unsatisfactory impact resistance, especially at a low temperature.
The above-mentioned process is also disadvantageous in that, to make the amount of the vinyl aromatic hydrocarbon compound, for example, styrene, copolymerized with butadiene in the butadiene prepolymer, it is necessary to either increase the ratio in the amount of butadiene to the vinyl aromatic hydrocarbon compound or to decrease the degree of the conversion of the butadiene to about 20%, and thus a large amount of non-polymerized butadiene must be removed from the prepolymerization system before the copolymerization process.
Japanese Examined Patent Publication (Kokoku) No. 41-6917 and U.S. Pat. No. 3,299,178 disclose a process for producing a high impact butadiene-vinyl aromatic compound copolymer in which butadiene is prepolymerized in the presence of a specific catalyst comprising an iodine-containing compound, for example, titanium tetraiodide and an organic metal compound in a vinyl aromatic compound medium. This process can provide a butadiene prepolymer, but is disadvantageous in that the prepolymerization rate of butadiene is extremely low, to an extent such that the conversion of butadiene at a degree of conversion of at least 50%, preferably at least 60%, is very difficult, and thus a large amount of non-prepolymerized butadiene must be removed from the prepolymerization system before the copolymerization step. Also, the above-mentioned process is disadvantageous in that the resultant copolymer exhibits a poor heat-degradation resistance, probably because the resultant copolymer contains a residue of the catalyst comprising the titanium compound and/or a derivative thereof.
Further, U.S. Pat. No. 4,575,538 discloses a process for producing a high impact butadiene-aromatic vinyl compound copolymer, in which, butadiene is prepolymerized in the presence of a specific catalyst composition comprising a rare earth metal halide-ligand complex (component A) formed by admixing a rare earth metal halide and an electron donor ligand and an organometal compound (component B). This specific catalyst has a low polymerization activity for butadiene in the vinyl aromatic compound medium, and therefore, it is very difficult to prepolymerize butadiene at a degree of conversion of at least 50%, preferably at least 60%, even when a large amount of the catalyst is used, and thus it is necessary to eliminate a large amount of non-prepolymerized butadiene from the prepolymerization system before the copolymerization step. Further the resultant copolymer from the above-mentioned process contains a large amount of gel-like polymeric substance which is insoluble in the vinyl aromatic compound medium, and thus exhibits a poor impact resistance and an uneven appearance.